Process for producing inorganic fine grains, inorganic fine grains, rare earth element-activated barium fluorohalide fluorescent substance, and radiation image conversion panel

ABSTRACT

A process for producing inorganic fine grains in a definite form having a small grain size, the inorganic fine grains obtained by this process, a rare earth element-activated barium fluorohalide fluorescent substance made using the grains, and a radiation image conversion panel with a layer of the fluorescent substance. The process features adding, to a solution containing an inorganic compound, a solid matter substantially insoluble in the solution, promoting crystallization or precipitation in the solution to form crystal or precipitate, and separating out the resulting crystal or precipitate. The inorganic fine grains produced by this process are represented by the formula BaFI:xLn (Ln represents at least one of Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm and Yb, and 0&lt;x≦0.2), have a cubic form and have a volume-average grain size of 1 to 10 μm.

BACKGROUND OF THE PRESENT INVENTION

[0001] 1. Field of the Present Invention

[0002] The present invention relates to a process for producinginorganic fine grains, to inorganic fine grains, to a rare earthelement-activated barium fluorohalide fluorescent substance, and to aradiation image conversion panel.

[0003] 2. Description of the Related Art

[0004] As a fluorescent substance for a radiation image conversion panelwhich is used in radiography, a divalent europium-activated bariumfluorohalide fluorescent substance (BaFX:Eu²⁺ wherein X is at least oneof Cl, Br and I; this is applied hereinafter) has been so far known.This fluorescent substance, when excited with radiation such as X-rays,electron rays or ultraviolet rays, allows near infrared luminescence(instantaneous luminescence) with maximum luminescence near 390 nm.

[0005] It has been further found that when this fluorescent substance isirradiated with the radiation and then excited with an electromagneticwave (excitation light) in a visible to infrared region, it allows nearultraviolet luminescence, namely stimulation luminescence. As describedin JP-A No. 55-12145, this fluorescent substance has attracted muchinterest as a fluorescent substance for a radiation image conversionpanel employed in a radiation image conversion method using stimulationof the fluorescent substance. Among others, a divalenteuropium-activated barium fluoroiodide fluorescent substance (BaFI:Eu²⁺) has a luminescence wavelength in a long wave side. Accordingly, therehas been a proposal that a semiconductor laser beam having anoscillation wavelength in a near ultraviolet region is used asexcitation light and this fluorescent substance is employed incombination therewith.

[0006] Barium fluoroiodide (BaFI) has been used as the divalenteuropium-activated barium fluoroiodide fluorescent substance or a rawmaterial for production of barium halide fluorescent substance withdivalent europium-activated iodine (so-called a fluorescent substanceraw powder).

[0007] For obtaining a barium fluorohalide, a method in which at leastone compound selected from the group consisting of barium carbonate,barium nitrate and barium sulfate is reacted with at least one compoundselected from the group consisting of hydrogen chloride, hydrogenbromide and hydrogen fluoride to form a barium halide and the product isthen reacted with hydrogen fluoride to form a barium fluorohalide hasbeen known. However, barium fluoroiodide is, unlike bariumfluorobromide, high in solubility in water. Therefore, the mere reactionof these materials in an aqueous medium is problematic in that theproduct cannot be obtained in satisfactorily high yield and impuritiestend to be incorporated.

[0008] Further, in consideration of the fact that the resulting crystalof barium fluoroiodide are used in a radiation image conversion panel,crystal in a cubic form having volume-average grain size of not morethan 10 μm are preferable. Thus, there is a demand for the developmentof a process to meet such requirements.

[0009] JP-A No. 7-233369 discloses a method in which BaX₂ is reactedwith an inorganic fluoride (for example, NH₄F) to produce a rare earthelement-activated barium fluorohalide (BaFX:Ln). JP-A No. 11-29324discloses the method described in JP-A No. 7-233369 is applied toproduction of barium fluoroiodide. Crystal of barium fluoroiodideobtained by this method are square (cubic), however, volume-averagegrain size (Dm) of those is not less than 10 μm. When the crystal isused in the existing radiation image conversion panel, graininess issometimes decreased.

[0010] In the method described in JP-A No. 10-140148, bariumfluoroiodide grains having a small volume-average grain size can beproduced, but the grain forms are indefinite. Accordingly, scattering ofexcitation light cannot be controlled in a radiation image conversionpanel, which might cause deterioration of an image quality.

[0011] Thus, a method for producing inorganic fine grains having adefinite form and a small grain size is sometimes required not only inobtaining the foregoing raw grains of the fluorescent substance but alsoin obtaining general inorganic fine grains.

SUMMARY OF THE PRESENT INVENTION

[0012] Under these circumstances, the present invention aims to providea process for producing inorganic fine grains in a definite form havinga small grain size, and inorganic fine grains formed by this process.Further, the present invention aims to provide a rare earthelement-activated barium fluorohalide fluorescent substance using atleast the inorganic fine grains formed by this process as a rawmaterial, and a radiation image conversion panel with an excellent imagequality using the rare earth element-activated barium fluorohalidefluorescent substance.

[0013] The foregoing aims are attained by the following approaches.

[0014] That is, a first aspect of a process for producing inorganic finegrains in the present invention is a process for producing inorganicfine grains, which comprises adding, to a solution containing at leastone inorganic compound, a solid matter substantially insoluble in thesolution, subjecting the solution to a procedure of promotingcrystallization or precipitation to form crystal or precipitate, andseparating the resulting crystal or precipitate.

[0015] A second aspect of the process for producing the inorganic finegrains in the present invention is the process for producing theinorganic fine grains according to the first aspect, in which theresulting crystal or precipitate is barium fluorohalide.

[0016] A third aspect of the process for producing the inorganic finegrains in the present invention is the process for producing theinorganic fine grains according to the first aspect, in which thesolution is a mixture of a BaI₂ aqueous solution containing at least onerare earth element and a fluoride aqueous solution, a Ba concentrationis not more than 3.0 mol/liter and a F/Ba molar ratio is not more than1.

[0017] A fourth aspect of the process for producing the inorganic finegrains in the present invention is the process for producing theinorganic fine grains according to the third embodiment, in which thefluoride aqueous solution is NH₄F aqueous solution.

[0018] A first aspect of inorganic fine grains in the present inventionis inorganic fine grains produced by adding, to a solution containing atleast one inorganic compound, a solid matter substantially insoluble inthe solution, subjecting the solution to a procedure of promotingcrystallization or precipitation to form crystal or precipitate, andseparating the resulting crystal or precipitate, in which the solutionis a mixture of BaI₂ aqueous solution containing at least one rare earthelement and a fluoride aqueous solution, a Ba concentration is not morethan 3.0 mol/liter and a F/Ba molar ratio is not more than 1, theinorganic fine grains being represented by the following basiccomposition formula (I), having a hexahedral form and having avolume-average grain size of 1 to 10 μm.

BaFI:xLn  (I)

[0019] wherein

[0020] Ln represents at least one of Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er,Tm and Yb, and

[0021] x represents a value of 0<x≦0.2.

[0022] A second aspect of the inorganic fine grains in the presentinvention is the inorganic fine grains according to the first aspect, inwhich the fluoride aqueous solution is an NH₄F aqueous solution.

[0023] A third aspect of the inorganic fine grains in the presentinvention is the inorganic fine grains according to the first aspect, inwhich an aspect ratio is 0.5 to 2.

[0024] A first aspect of a rare earth element-activated bariumfluorohalide fluorescent substance in the present invention is a rareearth element-activated barium fluorohalide fluorescent substance whichis produced using at least the inorganic fine grains according to thefirst aspect thereof.

[0025] A first aspect of a radiation image conversion panel in thepresent invention is a radiation image conversion panel in which a rareearth element-activated barium fluorohalide fluorescent substanceproduced using the inorganic fine grains according to the first aspectthereof is contained in a fluorescent substance layer.

BRIEF DESCRIPTION OF THE DRAWING

[0026]FIG. 1 is an electron micrograph of inorganic fine grains of thepresent invention obtained in Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention is described in detail below.

[0028] (A) Process for Producing Inorganic Fine Grains

[0029] The process for producing inorganic fine grains in the presentinvention is not particularly limited so long as it is a reaction in aliquid phase for obtaining crystal or precipitate.

[0030] First, a solution containing at least one inorganic compound isprepared. Then, a solid matter substantially insoluble in the solution(hereinafter sometimes referred to simply as a “solid matter”) is added.

[0031] The “solid matter substantially insoluble in the solution” meansa solid matter which has a corrosion resistance to the solution and asolvent constituting the solution and of which the component is noteluted in the solution. Further, it refers to a solid matter which isitself not reacted in the procedure of promoting crystallization orprecipitation of the solution and of which the component is thereforenot eluted in the solution.

[0032] The form of the solid matter is not particularly limited,powdery, square, cylindrical, disk-like, spherical, string-like,sheet-like form and the like. The form after the procedure of promotingcrystallization or precipitation does not necessarily have to keep theoriginal form unless the component constituting the solid matter iseluted in the solution. It is advisable to determine the grain diameterof the solid matter, as required, according to an amount of the solutionand the like. The volume-average grain size corresponding to thespherical form is preferably 0.1 to 30 mm, more preferably 1 to 10 mm.It is also advisable to determine the amount of the solid matter, asrequired, according to an amount of the solution and the like. It ispreferably 5 to 100% by mass, more preferably 10 to 50% by mass.

[0033] The solid matter used varies with the procedure of promotingcrystallization, precipitation, the solution used or the like. Examplesthereof include a zeolite, a Teflon zeolite (manufactured by Chemware),a Teflon lashing, a Teflon ball, a Teflon punching sheet, silica gel,PVDF (polyvinylidene fluoride) pellets, glass beads and a Teflon jointsealant (manufactured by Gore Tex) and the like Besides the foregoingsolid matters, solid matters made of a fluororesin having a highchemical stability, especially PTFE (polytetrafluoroethylene (tradename: Teflon)) or PVDF (polyvinylidene fluoride), silica and the likeare also available.

[0034] After the solid matter is added to the solution, the resultingsolution is subjected to the procedure of promoting crystallization orprecipitation to form crystal or precipitate.

[0035] As the procedure of promoting crystallization or precipitation, aconcentration method, a precipitation method and the like are listed.

[0036] The concentration method is a method in which a solution is atleast one of treated under reduced pressure and heated to precipitatecrystal. For example, it refers to a method in which a sodium chlorideaqueous solution is used as the solution, and this is supersaturatedthrough heating and the like to precipitate sodium chloride crystal.

[0037] The precipitation method is a method in which such a solution(substance) as to form a substance having a low solubility is added to amaterial dissolved in a solution to form a precipitate. For example, itrefers to a method in which a barium salt aqueous solution is used asthe solution, a sulfate compound is added thereto as a precipitatingagent, and heating and the like is conducted as required to obtain aprecipitate of barium sulfate. Further, the precipitation methodincludes a method in which an activator and a raw material of afluorescent substance are co-precipitated to obtain a precipitate and amethod in which Y₂O₃ and Eu₂O are dissolved with hydrochloric acid andco-precipitated as an oxalate with the addition of oxalic acid to obtaina precipitate as a raw material of a Y₂O₂S:Eu fluorescent substance. Itcan be applied to the process for producing the inorganic fine grains inthe present invention.

[0038] In combination with the procedure of promoting crystallization orprecipitation, treatment under at least one of reduced pressure and heattreatment, treatment of removing vapor on a liquid surface and the likeby suction, blowing of dry air or the like may be used.

[0039] The crystal or the precipitate obtained by the procedure ofpromoting crystallization or precipitation is separated from thesolution by a known method such as filtration (suction filtration orpressure filtration), centrifugation or the like. The separated crystalor the precipitate is subjected to washing, drying, classification andthe like, as required, to form the inorganic fine grains.

[0040] The process for producing the inorganic fine grains in thepresent invention is preferably applied to a process for producing arare earth element-activated barium fluoroiodide grains. The applicationof the process for producing the inorganic fine grains in the presentinvention to a process for producing a rare earth element-activatedbarium fluoroiodide grains is described below.

[0041] First, a BaI₂ aqueous solution containing at least one rare earthelement as an activator is mixed with a fluoride aqueous solution.

[0042] Examples of the rare earth element include Ce, Pr, Sm, Eu, Gd,Tb, Dy, Pr, Ho, Nd, Er, Tm and Yb. Eu and Ce are preferable. The contentof the activator in the BaI₂ aqueous solution is preferably 0.0001 to0.6 mol/liter, more preferably 0.001 to 0.1 mol/liter.

[0043] Further, for adjusting properties and the like of the final rareearth element-activated barium fluoroiodide grains, an additive(compound) containing an alkali metal, an additive (compound) containingan alkaline earth metal except Ba, a small amount of acid, ammonia,water-soluble polymer and water-insoluble metal oxide fine powder andthe like may be added.

[0044] Examples of the additive (compound) containing the alkali metalincludes salts (halide, nitrate, nitrite, acetate and the like) of Li,Na, K, Rb and Cs. In view of the reduction of impurities in theresulting grains, a halide containing a halogen element (F or I)incorporated in the final barium fluoroiodide is preferable. The amountof the additive containing the alkali metal is adjusted according to anecessary amount for the final composition.

[0045] Examples of the additive (compound) containing the alkaline earthmetal except Ba include salts (halide, nitrate, nitrite, acetate and thelike) of Ca, Sr and the like. In view of the reduction of impurities inthe resulting grains, a halide containing a halogen element incorporatedin the final barium fluoroiodide is preferable. The amount of theadditive containing the alkaline earth metal except Ba may be adjustedaccording to a necessary amount for the final composition. It ispreferably 20 to 5,000 ppm, more preferably 50 to 2,000 ppm in areaction mother liquor.

[0046] Moreover, the fluoride concentration in the fluoride aqueoussolution is preferably 1 to 10 mol/liter. As the fluoride, it ispreferable to use ammonium fluoride (NH₄F), alkali metal fluoride (LiF,NaF, KF or the like) and alkaline earth metal fluoride (MgF₂, CaF₂,SrF₂, BaF₂ or the like which may be in the form of a slurry). Of these,ammonium fluoride (NH₄F) is more preferable.

[0047] When the BaI₂ aqueous solution is mixed with the fluoride aqueoussolution, the Ba concentration is preferably not more than 3.0mol/liter, more preferably 2.0 to 2.6 mol/liter. When it exceeds 3.0mol/liter, the crystallization reaction occurs too early, and theresulting grain form might not be a hexahedron having an aspect ratio(0.5 to 2.0, preferably 0.8 to 1.5) to be described later.

[0048] It is preferable that the BaI₂ aqueous solution is mixed with thefluoride aqueous solution so that the F/Ba molar ratio is not morethan 1. When the F/Ba ratio exceeds 1, precipitation of barium fluoride(BaF₂) is accelerated, and it might be incorporated into BaFI grainsformed. It is more preferably 0.4 to 0.9.

[0049] After the mixing, the solid matter substantially insoluble inthis solution is added. The meaning of the “solid matter substantiallyinsoluble” is as described earlier. As the solid matter, a zeolite, aTeflon zeolite (manufactured by Chemware), a Teflon lashing, a Teflonball, a Teflon punching sheet, silica gel, PVDF (polyvinylidenefluoride) pellets, glass beads, a Teflon joint sealant (manufacturedGore Tex) and the like are preferable among those listed above.

[0050] The solid matter may be added to either the BaI₂ aqueous solutioncontaining at least one rare earth element or the fluoride aqueoussolution before mixing them.

[0051] After the addition of the solid matter, the procedure ofpromoting crystallization or precipitation is conducted to obtain thecrystal or the precipitate made of the inorganic fine grains in thehexahedral form having the volume average grain size of not more than 10μm. The onset of the form selectivity by the solid matter is unclear inmany points. It is presumable because a catalytic activity is exhibitedby pores of the solid matter, concave and convex on the surface of thesolid matter or the substituent present on the surface or the like.

[0052] The hexahedral form (hereinafter sometimes referred to simply asa “hexahedron”) in the present invention refers to a rectangular form ora cubic form of which the aspect ratio is 0.5 to 2.0, preferably 0.8 to1.5.

[0053] When the inorganic fine grains are barium fluorohalide crystal,the crystal belongs to a PbFCl (lead fluorochloride)-type tetragonalsystem. In this crystal system, atomic arrangements of an a axis and a baxis are equivalent, but that of a c axis is different. Accordingly, theaspect ratio of the crystal grains in this system is usually representedby a ratio (L′/L) of a length (L) of a side corresponding to the c axisto a length (L′) of a side corresponding to the a axis or the b axis.That is, when the aspect ratio is closer to 1, the form is a cubic form.When it is smaller than 1, the form is a tabular form. When it is largerthan 1, the form is a columnar form.

[0054] L and L′ can easily be measured with an electron microscope. Forexample, it is advisable that the “aspect ratio” is obtained bycalculating aspect ratios of respective crystal grains observed in anarea of 5 cm×5 cm of an electron micrograph (1,000× magnification) andaveraging them.

[0055] After the addition of the solid matter, a concentration orprecipitation method is preferably used as the procedure of promotingcrystallization or precipitation. Further, a reaction rate can beincreased by providing reduced pressure of, preferably, 100 hPa to 900hPa, more preferably, 200 hPa to 600 hPa with an aspirator and the like.When it is less than 100 hPa, a reaction solution is sucked by bumping.When it exceeds 900 hPa, the effect of reduced pressure is littlebrought forth to decrease the reaction rate.

[0056] The temperature in the procedure of promoting crystallization orprecipitation is preferably 20 to 100° C., more preferably 40 to 80C.When it is less than 20° C., the reaction proceeds slowly. Meanwhile,when it exceeds 100° C., boiling tends to occur, and the form of theresulting grains might not be fixed.

[0057] The thus-obtained precipitate is separated from the solutionafter the reaction by a known method such as filtration, centrifugationor the like.

[0058] After the separation, known washing, drying, classification andthe like are conducted, as required, to produce the rare earthelement-activated barium fluoroiodide grains which are inorganic finegrains.

[0059] The washing is conducted using an alcohol and the like such as2-propanol and the like as a solvent. The drying may be air drying, hotair drying, force-drying with an oven or vacuum drying.

[0060] The thus-obtained rare earth element-activated bariumfluoroiodide grains are, for example, represented by the following basiccomposition formula (I), have a cubic form, and have a volume-averagegrain size of 1 to 10 μm (preferably 2 to 7 μm).

BaFI:xLn  (I)

[0061] wherein

[0062] Ln represents at least one of Ce, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er,Tm and Yb, and

[0063] x represents a value of 0<x≦0.2.

[0064] Consequently, when the fluorescent substance produced from therare earth element-activated barium fluoroiodide grains by the processfor producing the rare earth element-activated barium fluorohalidefluorescent substance to be described later is used in the radiationimage conversion panel, the graininess is improved, and the scatteringof excitation light is controlled, making it possible to reducedeterioration of an image quality.

[0065] (B) Rare Earth Element-Activated Barium Fluorohalide FluorescentSubstance

[0066] The rare earth element-activated barium fluoroiodide grains canbe used as, for example, a raw material for production of a stimulationfluorescent substance (rare earth element-activated barium fluorohalidefluorescent substance) represented by the following basic compositionformula (II).

(Ba_(1-a)M^(II) _(a))FX·bM^(I)·cM^(III)·dA:xLn  (II)

[0067] wherein

[0068] M^(II) represents at least one alkaline earth metal selected fromthe group consisting of Sr, Ca and Mg,

[0069] M^(I) represents at least one alkali metal selected from thegroup consisting of Li, Na, K, Rb and Cs,

[0070] M^(III) represents a compound of at least one trivalent metalselected from the group consisting of Al, Ga, In, Tl, Sc, Y, Cd and Lu(except Al₂O₃),

[0071] X represents at least one halogen selected from the groupconsisting of Cl, Br and I,

[0072] Ln represents at least one rare earth element selected from thegroup consisting of Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Nd, Er, Tm and Yb,

[0073] A represents at least one metal oxide selected from the groupconsisting of Al₂O₃, SiO₂ and ZrO₂, and

[0074] a, b, c, d and x represent values of 0≦a≦0.3, 0≦b≦2,0≦c≦2,0≦d≦0.5 and 0<x≦0.2 respectively.

[0075] Further, the form of grains of the stimulation fluorescentsubstance includes a rectangular form, a regular hexahedral form, aregular octahedral form, an intermediate polyhedral form thereof and atetradecahedral form and the like. Of these, a tetradecahedral form ispreferable in that in the production of a radiation image conversionpanel, a less directional arrangement is provided in a fluorescentsubstance layer, undesirable lateral spread of excitation light andstimulation light is reduced and a sharpness of a radiation reproductionimage obtained is improved.

[0076] A process for producing the rare earth element-activated bariumfluorohalide fluorescent substance is described below.

[0077] The rare earth element-activated barium fluorohalide fluorescentsubstance (hereinafter sometimes referred to simply as a “fluorescentsubstance”) is produced, as described below, from [Fluorescent substanceraw materials] through steps, [Step of mixing raw materials], [Burningstep], [Cooling step] and, as required, [Other steps]. However, theprocess is not limited thereto.

[0078] [Fluorescent Substance Raw Materials]

[0079] With respect to fluorescent substance raw materials, crystalgrains of the rare earth element-activated barium fluoroiodide producedby the foregoing method of the present invention are used. Other rawmaterials are not particularly limited, and those obtained by any knownmethods are available.

[0080] As fluorescent substance raw materials, the following rawmaterials (1) to (5) can be listed.

[0081] (1) rare earth element-activated barium fluoroiodide produced bythe process for producing the inorganic fine grains in the presentinvention. Further, as required, at least one barium halide selectedfrom the group consisting of BaF₂, Ba Cl₂, BaBr₂, BaI₂, BaFBr, BaFI andBaFCl.

[0082] (2) at least one alkaline earth metal halide selected from thegroup consisting of CaF₂, CaCl₂, CaBr₂, CaI₂, SrF₂, SrCl₂, SrBr₂, SrI₂,MgF₂, MgCl₂, MgBr₂ and MgI₂.

[0083] (3) at least one alkali metal halide selected from the groupconsisting of CsCl, CsBr, CsI, NaCl, NaBr, NaI, KCl, KBr, KI, RbCl,RbBr, RbI, RbF, CsF, NaF, KF, LiF, LiCl, LiBr and LiI.

[0084] (4) at least one metal oxide selected from the group consistingof Al₂O₃, SiO₂ and ZrO₂.

[0085] (5) at least one compound selected from the group consisting ofcompounds (halide, oxide, nitrate, sulfate and the like) of rare earthelements (Ce, Pr, Sm, Eu, Gd, Tb, Dy, Pr, Ho, Nd, Er, Tm and Yb). Theraw material (5) is not indispensable, and when it is added, its amountmay be small.

[0086] Further, an ammonium halide (NH₄X′ in which X′ represents F, Cl,Br or I) and the like may be used as a flux.

[0087] [Step of Mixing Raw Materials]

[0088] Desired raw materials are optionally selected from among theforegoing raw materials (1) to (5), and stoichiometric amounts thereofare measured according to a desired composition ratio. They are mixed toprepare a mixture of raw materials of a fluorescent substance.

[0089] A method for preparing the mixture of the fluorescent substanceraw materials can properly be selected from among known mixing methods.For example, the mixture of the fluorescent substance raw materials maybe prepared by the following methods (i) to (iv).

[0090] (i) Method in which the amounts of the fluorescent substance rawmaterials (1) to (5) are measured and they are only mixed.

[0091] (ii) Method in which the amounts of the fluorescent substance rawmaterials (1) to (4) are measured and mixed, the mixture is heated at atemperature of not less than 100° C. for a few hours, and theheat-treated product is mixed with the fluorescent substance rawmaterial (5).

[0092] (iii) Method in which the fluorescent substance raw materials (1)to (5) are mixed, and the mixture is heated at a temperature of not lessthan 100° C. for a few hours.

[0093] (iv) Method in which the fluorescent substance raw materials (1)to (4) are mixed in a state of a suspension, the suspension is dried atan elevated temperature, preferably 50 to 200° C. by reduced pressuredrying, vacuum drying, spray drying or the like, and the resulting dryproduct is mixed with the fluorescent substance raw material (5).

[0094] Further, preferable variations of the method (iv) can include amethod (iv-2) in which the fluorescent substance raw materials (1) to(5) are mixed in a state of a suspension, and the suspension is dried, amethod (iv-3) in which the suspension containing the fluorescentsubstance raw materials (1) and (5) is heated at a temperature of,preferably 50 to 200° C., and then dried at an elevated temperature byreduced pressure drying, vacuum drying, spray drying or the like, andthe resulting mixture is mixed with the fluorescent substance rawmaterials (2) to (4), and a method (iv-4) in which, when conductingburning at least twice, the fluorescent substance raw materials (1) and(2) are mixed in a state of a suspension, the fluorescent substance rawmaterials (3) and (4) are added after the primary burning, thesuspension is dried at an elevated temperature, preferably 50 to 200° C.by reduced pressure drying, vacuum drying, spray drying or the like, andthe resulting dry product is mixed with the fluorescent substance rawmaterial (5), and the like.

[0095] Also available is a method for preparing a rare earthelement-activated alkaline earth metal fluorohalide stimulationfluorescent substance of a tetradecahedral form with a grain form and agrain aspect ratio controlled as described in JP-A Nos. 7-233369 and10-195431, namely, a method (v) using, in addition to the methods (i) to(iv-4) for preparing the mixture of the fluorescent substance rawmaterials, a procedure capable of imparting shear force in mixing thefluorescent substance raw materials, or a method (vi) using a procedurecapable of controlling conditions such as timing of addition and mixingof the fluorescent substance raw materials and the like.

[0096] A mixing unit used for the mixing in the methods (v) and (vi) canproperly be selected from among known mixing units such as variousmixers, a V-shaped blender, a ball mill, a rod mill and the like.

[0097] The following various additives can be added for improving anamount of stimulation luminescence, an erasability and the like inproducing the fluorescent substance.

[0098] Examples thereof can include B described in JP-A No. 57-23673, Asdescribed in JP-A No. 57-23675, tetrafluoroborate compounds described inJP-A No. 59-27980, hexafluoride compounds described in JP-A No.59-47289, transition metals such as V, Cr, Mn, Fe, Co, Ni and the likedescribed in JP-A No. 59-56480 and BeX″₂ (in which X″ represents atleast one halogen atom selected from the group consisting of F, Cl, Brand I) described in JP-A No. 59-75200.

[0099] When the additives are added, they are added and mixed either inmeasuring the amounts of the fluorescent substance raw materials andmixing the same or before burning.

[0100] [Burning Step]

[0101] The mixture of the fluorescent substance raw materials is filledin a heat-resistant container such as a quartz boat, an aluminacrucible, a quartz crucible, a core tube or the like, and placed in acore of an electric furnace to conduct burning.

[0102] The burning temperature is preferably 600 to 1,000° C., morepreferably 700 to 850° C. When the burning temperature is less than 600°C., generation of F+as a source of diffusion or stimulation of anactivator element in host crystal might be insufficient. When it exceeds1,000° C., host crystal might be melted.

[0103] The burning time varies with the amount of the mixture of thefluorescent substance raw materials, the burning temperature and thetemperature of drawing from the furnace. Generally, it is preferably 0.5to 6 hours, more preferably 1 to 3 hours.

[0104] When the burning time is less than 0.5 hour, generation of F⁺ asa source of diffusion or stimulation of an activator element in hostcrystal might be insufficient. Even when it exceeds 6 hours, theproperties of the fluorescent substance are little changed, and aproductivity might be decreased.

[0105] An atmosphere in the core tube at the time of burning ispreferably an atmosphere using a neutral or slightly oxidizing gas.

[0106] Examples of the neutral gas include inert gases such as He, Ne,Ar, N₂and the like.

[0107] The slightly oxidizing gas refers to a weakly oxidizing gas inwhich 100 to 100,000 ppm, preferably 150 to 50,000 ppm of oxygen iscontained in a unit volume of neutral gas. For example, a weakly acidicgas in which oxygen at the foregoing concentration is contained in aninert gas such as He, Ne, Ar, N₂ or the like is mentioned.

[0108] Further, it is preferable that a slow cooling step is providedbefore a cooling step to be described later as a post treatment afterburning the mixture of the fluorescent substance raw materials at afixed temperature as described above.

[0109] The slow cooling step may be conducted immediately after burningthe mixture of the fluorescent substance raw materials. It is preferablethat this step is conducted after the lapse of a fixed time while theremoval and the substitution of the atmosphere are conducted at a fixedtemperature.

[0110] In the slow cooling, the temperature is decreased uponcontrolling the temperature at a moderate temperature gradient until thetemperature reaches a predetermined temperature from the start-up.Especially in view of improving the luminescence of the stimulationfluorescent substance, it is preferable that the slow cooling isconducted to a temperature which is lower than a temperature incompleting the burning by 20 to 200° C. at a rate of temperature fall of0.2 to 5° C./min.

[0111] [Cooling Step]

[0112] The cooling in the cooling step may be conducted by a method inwhich a product is allowed to stand to decrease a temperature or amethod in which a temperature is forcibly decreased while beingcontrolled with a cooler. However, for shortening a cooling time andstably producing a stimulation fluorescent substance having satisfactoryproperties, a method in which cooling is conducted by controlling atemperature to a desired temperature is preferable.

[0113] [Other Steps]

[0114] Further, the stimulation fluorescent substance after the burningcan be subjected to, as required, general steps such as a washing step,a drying step, a screening step and the like.

[0115] The rare earth element-activated barium fluorohalide fluorescentsubstance in a powdery state can be obtained by the burning. Theresulting powdery fluorescent substance may be subjected to, asrequired, general steps in production of a fluorescent substance, suchas washing, drying, screening and the like.

[0116] (C) Radiation Image Conversion Panel

[0117] The thus-obtained rare earth element-activated bariumfluorohalide fluorescent substance can be used as a stimulationfluorescent substance contained in a fluorescent substance layer of aradiation image conversion panel. The radiation image conversion panelis described below.

[0118] The radiation image conversion panel basically comprises asubstrate and a fluorescent substance layer formed thereon. Thefluorescent substance layer comprises a binder for supporting thestimulation fluorescent substance in a dispersed state. The fluorescentsubstance layer can be formed on the substrate by, for example, thefollowing method.

[0119] First, the grains of the barium fluorohalide fluorescentsubstance and the binder are added to an appropriate solvent, and theseare fully mixed to form a coating solution in which the fluorescentsubstance grains are uniformly dispersed in the binder solution.

[0120] Typical examples of the binder of the fluorescent substance layercan include proteins such as gelatin and the like, polysaccharides suchas dextran and the like, natural macromolecular materials such as gumarabic, and synthetic macromolecular materials such as polyvinylbutyral, polyvinyl acetate, nitrocellulose, ethyl cellulose, vinylidenechloride-vinyl chloride copolymer, polyalkyl (meth) acrylate, vinylchloride-vinyl acetate copolymer, polyurethane, cellulose acetatebutyrate, polyvinyl alcohol, linear polyester and the like. Of thesebinders, nitrocellulose, polyurethane, linear polyester, polyalkyl(meth) acrylate, a mixture of nitrocellulose and linear polyester and amixture of nitrocellulose and a polyalkyl (meth) acrylate are especiallypreferable. These binders may be those crosslinked with a crosslinkingagent.

[0121] Examples of the solvent for forming the coating solution caninclude lower alcohols such as methanol, ethanol, n-propanol, n-butanoland the like; chlorine-containing hydrocarbons such as methylenechloride, ethylene chloride and the like; ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone and the like; esters oflower fatty acids and lower alcohols such as methyl acetate, ethylacetate, butyl acetate and the like; ethers such as dioxane, ethyleneglycol monoethyl ether, ethylene glycol monomethyl ether and the like;and mixtures thereof.

[0122] The mixing ratio of the binder and the fluorescent substance inthe coating solution varies with the properties of the desired radiationimage conversion panel and the type of the fluorescent substance and thelike. Generally, it is preferably 1:1 to 1:100 (weight ratio), morepreferably 1:8 to 1:40 (weight ratio).

[0123] The coating solution may contain various additives such as adispersing agent for improving a dispersibility of the fluorescentsubstance grains in the coating solution, a plasticizer for improving anadhesion between the binder and the fluorescent substance grains in thefluorescent substance layer after formation and the like. Examples ofthe dispersing agent used for this purpose can include phthalic acid,stearic acid, caproic acid and a lipophilic surfactant. Examples of theplasticizer can include phosphate esters such as triphenyl phosphate,tricresyl phosphate, diphenyl phosphate and the like; phthalate esterssuch as diethyl phthalate, dimethoxyethyl phthalate and the like;glycolate esters such as ethylphthalylethyl glycolate,butylphthalylbutyl glycolate and the like; polyesters of polyethyleneglycol and aliphatic dibasic acids, such as polyester of triethyleneglycol and adipic acid, polyester of diethylene glycol and succinic acidand the like; and the like.

[0124] The thus-obtained coating solution containing the fluorescentsubstance grains and the binder is then uniformly coated on the surfaceof the substrate to form a film of the coating solution. This coatingprocedure can be conducted by an ordinary coating unit such as a doctorblade, a roll coater, a knife coater or the like.

[0125] After the formation of the film, the film is dried to completeformation of the fluorescent substance layer on the substrate. Thethickness of the fluorescent substance layer varies with the propertiesof the desired radiation image conversion panel, the type of thefluorescent substance, the mixing ratio of the binder and thefluorescent substance and the like. It is usually 20 μm to 1 mm. Thislayer thickness is preferably 50 to 500 μm.

[0126] The fluorescent substance layer is not necessarily formed bydirectly coating the coating solution on the substrate as noted above.For example, it is also possible that the coating solution is coatedseparately on a glass plate, a metal plate or a sheet (temporarysubstrate) such as a plastic sheet and the like, and dried to form afluorescent substance layer, and the layer is then peeled off from thetemporary substrate and pressed on the substrate, or the substrate andthe fluorescent substance layer are adhered using an adhesive and thelike.

[0127] The fluorescent substance layer may be a single layer or alaminate of at least two layers. In case of the laminate, at least onelayer can be a layer containing the barium fluorohalide fluorescentsubstance. Further, in both of the single layer and the laminate,another stimulation fluorescent substance can be used in combinationwith the barium fluorohalide fluorescent substance.

[0128] The substrate can optionally be selected from various materialsused as a substrate of sensitization sheet in ordinary radiography orvarious known materials as a substrate of a radiation image conversionpanel. Examples of such materials can include plastic films such ascellulose acetate, polyester, polyethylene terephthalate, polyamide,polyimide, triacetate, polycarbonate and the like, metallic sheets suchas an aluminum foil, an aluminum alloy foil and the like, plain paper,baryta paper, resin-coated paper, pigment paper containing pigment suchas titanium dioxide and the like, paper sized with polyvinyl alcohol andthe like, and the like. However, in consideration of characteristics andhandling of the radiation image conversion panel as an informationrecording material, the especially preferable material of the substrateis a plastic film. The plastic film may contain light-absorbingsubstance such as carbon black and the like or light-reflectingsubstance such as titanium dioxide and the like. The former is asubstrate suited for a radiation image conversion panel having a highsharpness, and the latter is a substrate suited for a radiation imageconversion panel having a high sensitivity.

[0129] In a known radiation image conversion panel, for enhancing anadhesion between a substrate and a fluorescent substance layer orimproving a sensitivity or an image quality (sharpness and graininess)as a radiation image conversion panel, an adhesion-imparting layer isformed on a surface of a substrate on the side where a fluorescentsubstance layer is formed by coating at least one macromolecularmaterial such as gelatin or the like, a light-reflecting layer made of alight-reflecting material such as titanium dioxide or the like and alight-absorbing layer made of a light-absorbing substance such as carbonblack or the like is formed thereon.

[0130] Moreover, as described in JP-A No. 58-200200, fine surfaceirregularities may uniformly be formed on a surface of a substrate onthe side of a fluorescent substance layer (in case of forming anadhesion-imparting layer, a light-reflecting layer, a light-absorbinglayer or the like on a surface of a substrate on the side of afluorescent substance layer, the outer surface) for improving asharpness of the resulting image.

[0131] In an ordinary radiation image conversion panel, a transparentprotecting layer for protecting a fluorescent substance layer physicallyand chemically is formed on a surface of a fluorescent substance layeropposite to the side in contact with a substrate.

[0132] The transparent protecting layer can be formed by a method inwhich a solution formed by dissolving a transparent macromolecularsubstance in an appropriate solvent is coated on a surface of afluorescent substance layer, examples of the transparent macromolecularsubstance being cellulose derivatives such as cellulose acetate,nitrocellulose and the like and synthetic macromolecular materials suchas polymethyl methacrylate, polyvinyl butyral, polyvinyl formal,polycarbonate, polyvinyl acetate, vinyl chloride-vinyl acetate copolymerand the like. Alternatively, it can also be formed by a method in whicha transparent thin film separately formed from polyethyleneterephthalate, polyethylene, polyvinylidene chloride, polyamide or thelike is adhered to a surface of a fluorescent substance layer using anappropriate adhesive. A thickness of the thus-formed transparentprotecting layer is preferably approximately 3 to 20 μm.

[0133] As described in JP-A No. 55-163500 and 57-96300, the radiationimage conversion panel may be colored with a colorant to improve asharpness of an image obtained by coloration. Further, as described inJP-A No. 55-146447, a white powder may be dispersed in the fluorescentsubstance layer for the same purpose.

EXAMPLES

[0134] The present invention is illustrated specifically below byreferring to Examples. However, the present invention is not limited tothese Examples.

Example 1

[0135] An aqueous solution (150 ml) containing 4 mol/liter of BaI₂ wasadded to a 300 ml separable beaker, and 3 ml of a solution containing0.1 mol/liter of EuI₃ and 47 ml of water were further added thereto.With stirring, 50 ml of an aqueous solution containing 6 mol/liter ofNH₄F was added to form a mixed solution which was maintained at 80° C.

[0136] To the mixed solution was added 50 g of a Teflon zeolite(manufactured by Chemware: boiling stone; a material, a form and thelike are shown in Table 1) as a solid matter. While the inside of theseparable beaker was exhausted with an aspirator, the reaction wasconducted for 2 hours with stirring to form a precipitate of BaFI:Eugrains (rare earth element-activated barium fluoroiodide grains) asinorganic fine grains.

[0137] Since excessive exhaustion results in bumping, this reaction wasconducted under reduced pressure of 533 hPa (40 cmHg) by providing aclearance in an exhaust tube portion.

[0138] After the reaction, the Teflon zeolite was separated with astainless mesh having an opening of 1 mm, and the precipitate wasfurther separated through suction filtration with a filter paper. Theprecipitate separated was uniformly sprayed 300 ml of IPA (isopropylalcohol) to wash, and vacuum-dried at 150° C. for 2 hours to obtain 83 gof BaFI:Eu grains.

[0139] With respect to the resulting BaFI:Eu grains, (1) observation ofa form, (2) measurement of an aspect ratio and (3) measurement of avolume average grain diameter were conducted as follows.

[0140] (1) Observation of a Form

[0141] The form of the BaFI:Eu grains was observed from a photographobtained using a scanning electron microscope (JSM-5400LV, manufacturedby JEOL Ltd.). It was found from the photograph that the BaFI:Eu grainshad a cubic form.

[0142] (2) Measurement of an Aspect Ratio

[0143] The aspect ratio of the BaFI:Eu grains was measured by obtaininga length (L′) of a side corresponding to an a axis or a b axis and alength (L) of a side corresponding to a c axis on each of the BaFI:Eugrains from a photograph (range: 5 cm×5 cm) obtained with the foregoingscanning electron microscope (1,000× magnification), calculating L′/Lratios thereof and averaging the same.

[0144] In this Example, the aspect ratio of the BaFI:Eu grains was 1. Itwas found, as in the observation of the form, that the BaFI:Eu grainshad a cubic form.

[0145] (3) Measurement of a Volume-Average Grain Size

[0146] The volume-average grain size was measured by a volume standardmode using a laser diffraction-type grain size distribution measuringdevice (LA-500 manufactured by Horiba Ltd.). The volume-average grainsize of the resulting BaFI:Eu grains was 6.5 μm.

[0147] BaFBr:Eu tetradecahedral grains (volume-average grain sizeapproximately 5 μm) were produced by the method described in Example 1of JP-A No. 7-233369. The BaFBr:Eu grains and the BaFI:Eu grains werefully mixed with a mixer at a Br to I composition ratio (molar ratio) of85:15 to form a mixture. At this time, 0.5% by weight of alumina finegrains was added for preventing sintering in burning.

[0148] 100 g of the thus-formed mixture was charged into a quartz boat,and burned using a burning furnace having a quartz core tube. Theburning was conducted in a trace oxygen atmosphere at a burningtemperature of 850° C. for a burning time of 2 hours. After the burning,the core tube was withdrawn from a heater portion, and cooled to roomtemperature while being vacuum-exhausted. After the cooling, 100 g ofmethanol was added, and the mixture was stirred for 3 hours. Then, theproduct was loosened through a nylon mesh having an opening of 20 μm,and classified. The product passed through the mesh was subjected tosolid-liquid separation with a filter paper, and dried with hot air toobtain a rare earth element-activated barium fluorohalide fluorescentsubstance (BaF(Br_(0.85)I_(0.15)):Eu grains) as a stimulationfluorescent substance.

[0149] Next, a step of producing a radiation image conversion panel isdescribed.

[0150] The barium fluorohalide fluorescent substance (356 g), 15.8 g ofpolyurethane resin (Desmolac 4125 manufactured by Sumitomo BayerUrethane K.K.) and 2.0 g of bisphenol A-type epoxy resin were added tomethyl ethyl ketone as a solvent, and dispersed with a propeller mixerto form a coating solution having a viscosity of 3.0 Pa·s. This coatingsolution was coated on a polyethylene terephthalate film having anundercoat with a doctor blade, and then dried at 100° C. for 15 minutesto form a fluorescent substance layer having a thickness of 200 μm.

[0151] Subsequently, 70 g of a fluoroolefin-vinyl ether copolymer(Lumiflon LF100 manufactured by Asahi Glass Company, Ltd.) as afluororesin, 25 g of an isocyanate (Desmodur Z4370 manufactured bySumitomo Bayer Urethane K. K.) as a crosslinking agent, 5 g of abisphenol A-type epoxy resin and 10 g of a silicone resin fine powder(KMP-590 manufactured by The Shin-etsu Chemical Industry Co., Ltd.,volume-average grain size 1.2 μm) were added to a toluene-isopropylalcohol (1:1) solvent mixture to form a coating solution.

[0152] This coating solution was coated on the fluorescent substancelayer previously formed above using a doctor blade, then heat-treated at120° C. for 30 minutes for thermosetting, and dried to form a protectinglayer having a thickness of 10 μm. Thus, a radiation image conversionpanel having a stimulation fluorescent substance layer 200 μm inthickness was produced.

Examples 2 to 7

[0153] Inorganic fine grains were produced as in Example 1 except thatmaterials shown in Table 1 were used instead of the Teflon zeolite inExample 1. Further, a fluorescent substance was produced as inExample 1. The inorganic fine grains were subjected to the observationof the form, the measurement of the aspect ratio and the measurement ofthe volume-average grain size as in Example 1. The results are shown inTable 2.

[0154] An electron micrograph (1,000× magnification) of the BaFI:Eugrains obtained in Example 3 as inorganic fine grains is shown inFIG. 1. From FIG. 1, it was identified that all of the grains (crystal)had a grain size of 10 μm or less and had a cubic form.

[0155] Further, radiation image conversion panels were produced underthe same conditions as in Example 1 using the fluorescent substancesobtained in respective Examples.

Comparative Example 1

[0156] A mixed solution was formed as in Example 1 except that anordinary beaker was used instead of the separable beaker. A precipitateof inorganic fine grains (BaFI:Eu grains) was formed by spontaneousevaporation (5 hours) without adding the solid matter to the mixedsolution. In the same manner as in Example 1, the precipitate wasseparated from the mixed solution, washed, and dried to form inorganicfine grains. Further, a fluorescent substance was produced as inExample 1. The inorganic fine grains were subjected to the observationof the form, the measurement of the aspect ratio and the measurement ofthe volume-average grain size as in Example 1. The results are shown inTable 2.

[0157] Further, a radiation image conversion panel was produced underthe same conditions as in Example 1 using the fluorescent substanceobtained in this Comparative Example. (Comparative Example 2) Inorganicfine grains were produced as in Example 1 except that the solid matterwas not added. Further, a fluorescent substance was produced as inExample 1. The inorganic fine grains were subjected to the observationof the form, the measurement of the aspect ratio and the measurement ofthe volume-average grain size as in Example 1. The results are shown inTable 2.

[0158] Further, a radiation image conversion panel was produced underthe same conditions as in Example 1 using the fluorescent substanceobtained in this Comparative Example.

[0159] With respect to the radiation image conversion panels produced inExamples 1 to 7 and Comparative Examples 1 and 2, evaluation of an imagequality was conducted in the following manner.

[0160] First, X-rays were applied (tube voltage: 80 kVp, dose: 2.58×10⁻⁷(C/kg)(=1 mR)) from the side of the fluorescent substance layer formedon each of the radiation image conversion panels, and a laser beam of660 nm was then applied to read an image information. Regarding theimage quality of the image information read, DQE (detection quantumefficiency) was calculated. The value obtained in Comparative Example 1was rated as 100, and relative values obtained in Examples 1 to 7 andComparative Example 2 were compared. The results are shown in Table 2.

[0161] Incidentally, DQE was calculated by a method described in“Lifetime text 1 Clinical Imaging 1 pp. 103-104” (compiled by The JapanAssociation of Radiological Technologists (1991), Maguburosu Shuppan).TABLE 1 Name of Material of Form of size of Surface condition a solid asolid a solid a solid of a solid matter Maker matter matter mattermatter Example Teflon Chemware PTFE angular 5-10 mm many spherical 1zeolite indefinite protrusions of form 1.2 μm Example Teflon ChemwareTVDF hollow height no (smooth) 2 lashing cylindrical 6 mm form ExamplePVDF Aldrich PVDF disk form diameter 6 mm no (smooth) 3 thickness 2 mmExample silica gel Ikeda Rika SiO₂ spherical diameter 3-4 mm havingpores of 4 K.K. form submicron Example boiling Kanto Si0₂ + Al₂O₃indefinite ca. 6 mm having pores of 5 tips Kagaku small stone submicronK.K. form (containing water of crystallization) Example Teflon IkedaRika PTFE porous sheet Sheet 3 mm in no (smooth) 6 punching K.K. formthickness and 3 mm in sheet pore diameter is cut to a square of 5 mm.Example Gore Tex Gore Tex PTFE string form diameter 6.4 mm, fine fibersof 7 joint length 5 mm submicron sealant

[0162] TABLE 2 Volume- Image average quality Aspect grain size (relativeForm ratio (μm) DQE) Example 1 cubic 1.0 6.5 110 Example 2 cubic 1.1 6.5108 Example 3 cubic 1.0 5.3 112 Example 4 cubic 1.1 7.2 107 Example 5cubic 1.0 6.8 105 Example 6 cubic 1.2 7.5 107 Example 7 cubic 1.1 6.0110 Comparative cubic 1.3 15.3 100 Example 1 Comparative tabular 0.217.5 95 Example 2

[0163] In Examples 1 to 7, the solid matter was added in the productionof the inorganic fine grains (rare earth element-activated bariumfluoroiodide grains). Accordingly, the inorganic fine grains in a cubicform having the volume-average grain size of not more than 10 μm couldbe produced. Further, the image quality (relative DQE) of the radiationimage conversion panel produced by using the inorganic fine grains wasexcellent in comparison with Comparative Examples 1 and 2.

[0164] According to the process for producing the inorganic fine grainsin the present invention, the inorganic fine grains in a definite formhaving a small grain size can be produced. Further, the radiation imageconversion panel produced from the rare earth metal-activated bariumfluorohalide fluorescent substance using the inorganic fine grains isexcellent in image quality.

What is claimed is:
 1. A process for producing inorganic fine grainscomprising the steps of: adding to a solution, which includes at leastone inorganic compound, a solid matter which is substantially insolublein the solution; promoting one of crystallization and precipitation inthe solution, to produce one of crystal and precipitate; and separatingout the one of crystal and precipitate.
 2. The process for producinginorganic fine grains according to claim 1, wherein the solid mattercomprises at least a material selected from the group consisting ofpolytetrafluoroethylene, polyvinylidene fluoride, silica and alumina. 3.The process for producing inorganic fine grains according to claim 1,wherein the solid matter comprises an addition amount thereof of 5 to100% by mass relative to the solution.
 4. The process for producinginorganic fine grains according to claim 1, wherein the solid mattercomprises a volume-average grain size of 0.1 to 30 mm.
 5. The processfor producing inorganic fine grains according to claim 1, wherein thestep of promoting one of crystallization and precipitation includes atemperature of 20 to 100° C.
 6. The process for producing inorganic finegrains according to claim 1, wherein the one of crystal and precipitateproduced comprises barium fluorohalide.
 7. The process for producinginorganic fine grains according to claim 1, wherein the one of crystaland precipitate produced comprises barium fluoroiodide.
 8. The processfor producing inorganic fine grains according to claim 1, wherein thesolution comprises a mixture of a BaI₂ aqueous solution, which containsat least one rare earth element, and a fluoride aqueous solution, a Baconcentration in the solution being not more than 3.0 mol/liter, and aF/Ba molar ratio being not more than
 1. 9. The process for producinginorganic fine grains according to claim 8, wherein the at least onerare earth element comprises a rare earth element selected from thegroup consisting of Ce, Pr, Sm, Eu, Gd, Tb, Dy, Pr, Ho, Nd, Er, Tm andYb.
 10. The process for producing inorganic fine grains according toclaim 8, wherein the fluoride aqueous solution comprises a fluorideselected from the group consisting of NH₄F, LiF, NaF, KF, MgF₂, CaF₂,SrF₂ and BaF₂.
 11. The process for producing inorganic fine grainsaccording to claim 8, wherein the fluoride aqueous solution comprisesNH₄F aqueous solution.
 12. Inorganic fine grains, comprising inorganicfine grains which are represented by the following basic compositionformula (I), have a hexahedral form, and have a volume-average grainsize of 1 to 10 μm: BaFI:xLn  (I) in which Ln represents at least oneelement selected from the group consisting of Ce, Pr, Sm, Eu, Tb, Dy,Ho, Nd, Er, Tm and Yb, and x represents a value in the range 0<x≦0.2,wherein the inorganic fine grains are produced by adding, to a solutioncontaining at least one inorganic compound, a solid matter substantiallyinsoluble in the solution, promoting crystallization or precipitation inthe solution to produce crystal or precipitate, and separating out theresulting crystal or precipitate, the solution including a mixture of aBaI₂ aqueous solution, which contains at least one rare earth element,and a fluoride aqueous solution, a Ba concentration in the solutionbeing not more than 3.0 mol/liter and a F/Ba molar ratio being not morethan
 1. 13. The inorganic fine grains according to claim 12, wherein thefluoride aqueous solution comprises NH₄F aqueous solution.
 14. Theinorganic fine grains according to claim 12, wherein the inorganic finegrains comprise an aspect ratio of 0.5 to
 2. 15. A rare earthelement-activated barium fluorohalide fluorescent substance, wherein therare earth element-activated barium fluorohalide fluorescent substanceis produced using at least inorganic fine grains produced by adding to asolution a solid matter substantially insoluble in the solution,promoting crystallization or precipitation in the solution to producecrystal or precipitate, and separating out the resulting crystal orprecipitate, the solution including a mixture of a BaI₂ aqueoussolution, which contains at least one rare earth element, and a fluorideaqueous solution, a Ba concentration in the solution being not more than3.0 mol/liter, a F/Ba molar ratio being not more than 1, and theinorganic fine grains being represented by the following basiccomposition formula (I), having a hexahedral form and having avolume-average grain size of 1 to 10 μm: BaFI:xLn  (I) wherein Lnrepresents at least one element selected from the group consisting ofCe, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm and Yb, and x represents a valuein the range 0<x≦0.2.
 16. The rare earth element-activated bariumfluorohalide fluorescent substance according to claim 15, wherein therare earth element-activated barium fluorohalide fluorescent substancecomprises a stimulation fluorescent substance represented by thefollowing basic composition formula (II): (Ba_(1-a)M^(II)_(a))FX·bM^(I)·cM^(III)·dA:xLn  (II) wherein M^(II) represents at leastone alkaline earth metal selected from the group consisting of Sr, Caand Mg, M^(I) represents at least one alkali metal selected from thegroup consisting of Li, Na, K, Rb and Cs, M^(III) represents at leastone compound of a trivalent metal selected from the group consisting ofAl, Ga, In, Tl, Sc, Y, Cd and Lu, except Al₂O₃, X represents at leastone halogen selected from the group consisting of Cl, Br and I, Lnrepresents at least one rare earth element selected from the groupconsisting of Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Nd, Er, Tm and Yb, Arepresents at least one metal oxide selected from the group consistingof Al₂O₃, SiO₂ and ZrO₂, and a, b, c, d and x represent values in theranges 0≦a≦0.3, 0≦b≦2, 0≦c≦2, 0≦d≦0.5 and 0<x≦0.2, respectively.
 17. Aradiation image conversion panel comprising, on a substrate, at leastone fluorescent substance layer including at least a rare earthelement-activated barium fluorohalide fluorescent substance containingat least inorganic fine grains that are represented by the followingbasic composition formula (I), have a hexahedral form and have avolume-average grain size of 1 to 10 μm: BaFI:xLn  (I) in which Lnrepresents at least one element selected from the group consisting ofCe, Pr, Sm, Eu, Tb, Dy, Ho, Nd, Er, Tm and Yb, and x represents a valuein the range 0<x≦0.2, wherein the inorganic fine grains are produced byadding to a solution a solid matter substantially insoluble in thesolution, promoting crystallization or precipitation in the solution toproduce crystal or precipitate, and separating out the resulting crystalor precipitate, the solution including a mixture of a BaI₂ aqueoussolution, which contains at least one rare earth element, and a fluorideaqueous solution, a Ba concentration in the solution being not more than3.0 mol/liter, and a F/Ba molar ratio being not more than
 1. 18. Theradiation image conversion panel according to claim 17, wherein thefluorescent substance layer comprises a thickness of 20 μm to 1 mm.